[llvm] [GlobalISel] Handle div-by-pow2 (PR #83155)

[Pierre van Houtryve via llvm-commits]

================
@@ -5286,6 +5286,141 @@ MachineInstr *CombinerHelper::buildSDivUsingMul(MachineInstr &MI) {
   return MIB.buildMul(Ty, Res, Factor);
 }
 
+bool CombinerHelper::matchSDivByPow2(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_SDIV && "Expected SDIV");
+  if (MI.getFlag(MachineInstr::MIFlag::IsExact))
+    return false;
+  auto &SDiv = cast<GenericMachineInstr>(MI);
+  Register RHS = SDiv.getReg(2);
+  auto MatchPow2 = [&](const Constant *C) {
+    if (auto *CI = dyn_cast<ConstantInt>(C))
+      return CI->getValue().isPowerOf2() || CI->getValue().isNegatedPowerOf2();
+    return false;
+  };
+  return matchUnaryPredicate(MRI, RHS, MatchPow2, /* AllowUndefs= */ false);
+}
+
+void CombinerHelper::applySDivByPow2(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_SDIV && "Expected SDIV");
+  auto &SDiv = cast<GenericMachineInstr>(MI);
+  Register Dst = SDiv.getReg(0);
+  Register LHS = SDiv.getReg(1);
+  Register RHS = SDiv.getReg(2);
+  LLT Ty = MRI.getType(Dst);
+  LLT ShiftAmtTy = getTargetLowering().getPreferredShiftAmountTy(Ty);
+
+  Builder.setInstrAndDebugLoc(MI);
+
+  unsigned Bitwidth = Ty.getScalarSizeInBits();
+  auto Zero = Builder.buildConstant(Ty, 0);
+
+  auto RHSC = getConstantOrConstantSplatVector(RHS);
+  if (RHSC.has_value()) {
----------------
Pierre-vh wrote:

```suggestion
  if (auto RHSC = getConstantOrConstantSplatVector(RHS)) {
```

https://github.com/llvm/llvm-project/pull/83155